Marine seismic exploration typically employs a seismic acquisition system to acquire seismic data. The seismic acquisition system includes a source, which initiates a seismic wave, sensors, which detect the seismic wave, and other components. The seismic wave propagates from the source through the water and subsurface where it illuminates subsea geologic formations. As it illuminates interfaces or boundaries, part of the seismic wave is returned or reflected back through the earth in the up-going direction as a primary reflection. A portion of this reflected seismic wave is detected by the sensors of the seismic acquisition system, converted into electrical signals, and recorded as seismic data for subsequent processing. An analysis of these recorded data or signals makes it possible to estimate the structure, position, impedance, fluid type, and lithology of subsea geologic formations, among other parameters, thereby completing an important step in the exploration process.
FIG. 1 shows a simplified example of a typical marine seismic acquisition system. A first ship 1 tows a seismic source 2 several meters below the surface 3 of the ocean. The seismic source 2 is activated to produce a down-going seismic wave 4d that is at least partially reflected by a subsea interface or boundary 5. The up-going reflected seismic wave 4u then travels toward the sensors 6 and is measured.
The sensors 6 used in typical marine seismic exploration include pressure sensors and/or motion (displacement, velocity, acceleration, or higher temporal derivatives of displacement) sensors. Typically, pressure sensors are hydrophones and motion sensors are geophones that usually measure particle velocity or acceleration. Hydrophones measure a scalar pressure and are not sensitive to the propagation direction of a seismic wave. Geophones, which might be vertically oriented, provide a measurement in the direction of orientation whose polarity depends on whether the direction of propagation is up-going or down-going. The amplitude of a geophone response is also related to the angle of the propagation relative to the sensitive direction of a geophone. If a seismic wave is recorded by a hydrophone and a vertically oriented geophone that have identical impulse responses, then a polarity comparison between the hydrophone and geophone recordings can determine whether the wave is propagating in the up-going or down-going direction. Hydrophones and geophones disposed at the seafloor are typically used in pairs when collecting seismic data. However, hydrophones, unlike geophones, can measure seismic data anywhere in the water column. This invention applies to motion sensors positioned anywhere in the water column including the seafloor or surface.
In another type of marine seismic surveying, the sensors 6 are located at regular intervals in one or more ocean bottom cables (OBC) 7 that are arranged on the seafloor 9. When necessary, a second ship 8 is used to move the OBC 7 to a new position on the seafloor 9. Several miles of OBC 7 are typically deployed along the seafloor 9, and several OBCs are typically deployed in parallel arrangements. OBC 7 arrangements are particularly well suited for use in certain zones (such as zones cluttered with platforms or where the water is very shallow) and where the use of ship-towed hydrophone arrays (which are located proximate the ocean surface 3 and are typically referred to as “streamers”) are not practical. In another type of seismic surveying, sensor packages, often containing a hydrophone and one or several geophones, are deployed on the ocean bottom as separate stations. A combination of separate ocean bottom stations and ocean bottom cables can be deployed. The geophone (motion sensor) and hydrophone (pressure sensor) might be connected to a recording system typically on a vessel. FIG. 2 shows another type of marine seismic surveying. A marine cable or streamer 21 incorporating pressure sensing hydrophones is designed for continuous towing through the water. A marine streamer 21 might be made up of a plurality of active or live hydrophone arrays 23 separated by spacer or dead sections 25. Usually the streamers are nearly neutrally buoyant and depth controllers 27 or depressors are attached to depress the streamer 21 to the proper towing depth. A tail buoy with a radar reflector 29 is typically attached to the end of the streamer. The entire streamer may be 3-6 Km in length and may be towed by a ship 31.
A reflection off the ocean surface gives rise to what is called a ‘ghost’. When an isotropic source is fired a down-going ‘source ghost’ combines with the wave initially radiated in the down-going direction. This combination of two down-going waves modulates the source's amplitude spectrum by the amplitude of a sine function and results in the attenuation of low frequencies for the combined down-going wave and a reduction in the amount of information available in the seismic data. This loss of low frequencies is observed in a measurement of the combined wave's pressure and its particle motion. This invention describes sources wherein the combined down-going wave will not have its amplitude spectrum modulated in this way but instead will modulate the source's amplitude spectrum by the amplitude of a cosine function to enhance the acquisition of low frequency seismic data.
Recorded seismic data also includes a ‘sensor ghost’. In the sensor case, an up-going wave reflects off the ocean surface and gives rise to a down-going sensor ghost. The sensor measures the combined up-going wave and down-going sensor ghost. This contrasts with the source case, which is a combination of two down-going waves. As an example, in a one-dimensional OBC case, a sensor ghost can be reflected off the ocean surface and delayed by the two-way travel time in the water column. The sensor ghost has the same effect on a pressure measurement as observed for the source ghost because pressure is a scalar quantity and it is not sensitive to direction. This means that a measurement of the low frequencies associated with the combined wave's pressure will be attenuated in a similar way as they would be for the source case. However, motion is a vector quantity sensitive to direction and the measurement of the combined wave's motion exhibits different behavior from that associated with the combined wave's pressure. The low frequencies associated with the combined wave's motion are retained and enhanced. Mathematically, the amplitude spectrum associated with the combined wave is modulated by the amplitude of a sine or cosine function depending on whether the wave's pressure or motion, respectively, is being measured.
When low frequencies are not available in the seismic data, impedance estimates for the subsurface derived from the seismic data will not contain low frequency information. Without low frequency information, unique solutions for impedance are not available. This hampers interpretation of the derived impedance because many unconstrained solutions are possible. Some of the impedance solutions might accurately describe the earth's impedance and others will not. If the low frequencies are retained, additional information can be incorporated into the impedance solution to reduce the nonuniqueness and to restrict attention to only the most geophysically plausible structures.
A number of methods for inserting low frequencies in the 0-10 Hz band have been developed. Some researchers have resorted to using the measured impedance at a nearby well (Galbraith, J. M., and Millington, G. F., 1979, Low frequency recovery in the inversion of seismograms: Journal of the CSEG, v. 15, p. 30-39). Another approach is to add low frequencies derived from velocity analysis (Layergne, M., and Willm, C., 1977, Inversion of seismograms and pseudovelocity logs: Geophysical. Prosp., v. 25, p. 231-250). There are also some methods by which the missing low frequency information can be estimated. One such method requires finding a reflectivity function made of isolated delta functions and another method requires predicting the missing frequencies from the band limited reflectivity function (Oldenburg, D. W., Scheuer, T., and Levy, S., 1983, Recovery of the acoustic impedance from reflection seismograms: Geophysics, v. 48, p. 1318-1337).
A major source of the lack of low frequency content in seismic data resides with the seismic acquisition system. As shown in FIG. 3, the monopole marine source array 31 radiates a wave having frequency spectrum A(f). The ocean surface 3 creates a reflection (or ghost) with frequency spectrum −A(f) that combines with the original down-going wave having the frequency spectrum A(f). The composite down-going wave (down-going plus the ocean surface reflected wave) has an amplitude spectrum with attenuated low frequencies because of destructive interference at the low frequencies. The source's amplitude spectrum will be attenuated by the amplitude of a sine function 33.
The lack of low frequency content in the seismic wave is compounded when the up-coming reflections from the subsurface are recorded by a hydrophone 35 on the surface of the seafloor 36. Hydrophone measurements are subjected to a sensor ghost that modulates the amplitude spectrum by the amplitude of a sine function 35, which attenuates the low frequencies. Geophone measurements are subjected to a sensor ghost that modulates the amplitude spectrum by the amplitude of a cosine function 37.
Even when data are recorded by a geophone on an Ocean Bottom Cable (OBC) 37, modem OBC acquisition techniques still use a source that lacks low frequency content. As described above, a principal problem in the recovery of impedance from seismograms is this lack of low frequencies in the seismic data. Accordingly, there is a need for a method and apparatus which generates low frequencies which can be measured in the seismic data. This invention satisfies these needs.